In the case of a radar system using a “solid state” emission system, (a “solid state” emitter uses devices, in particular transistors, in which the current flows in semi-conducting materials which are solid as opposed to a vacuum tube emitter where the current consists of a confined electron beam which consists of an electron “gas”) the radar signals generally use a succession of pulses of duration T1. Moreover they must exhibit the highest possible ratio between the mean power emitted and maximum power emitted, thus it is necessary to have long pulses. Indeed, the cost of solid-state emission systems depends mainly on the maximum power, but the range of the radar depends by contrast mainly on the mean power.
One of the drawbacks related to the use of long pulses, in the conventional case of mono-static radars, is the presence of zones in which the radar cannot locate objects. Indeed, beyond about ten Watts of emitted power, it is impossible to emit and to receive simultaneously, except by using two separate antennas, this being expensive and generally very difficult to do for airborne systems constrained by size. It will not therefore be possible to receive a reflected signal during the emission of the emitted signal. For example, if the object causing the reflection is close by, the reflected signal arrives too early (the radar is still in the process of emitting) and the reception of this reflected signal will be impossible. Another zone in which the radar cannot locate any object is situated at the end of the location zone. Indeed, in this case a reflected pulse will return to the emitter at the same time as the next pulse emitted. The most troublesome zone in which the radar cannot locate any object is that which is situated at short range. Indeed, the masked zone situated at the end of the domain is not very troublesome since it suffices to increase the pulse repetition period (PRI) if it is desired to observe pulses reflected in this distance domain.
For pulses of duration T1, if the shape factor of the radar is h=T1/PRI, then the unusable spatial domain (geographical zone in which the radar is not capable of locating an object) is the zone situated at a distance of less than the distance travelled by a pulse during the time T1.
Systems are known which use emitted pulses which are not modulated, and in this case, it is possible to partially use the fraction of the pulse received which does not lie in the reception cutoff caused by the emission of the said pulse. Thus by using these systems it is possible to use the totality of the distance domain. On the other hand, the sensitivity is reduced in the incomplete zones. This sensitivity is all the more limited as the masking becomes more significant.
In order to improve the resolution of the locating system it is known to use a radar signal consisting of pulses containing a particular code so as to increase the spread band of the frequencies B of the signal emitted for a given pulse duration. If the compression caused by the particular code is ideal, the energy of the initial pulse of duration T1 is preserved but is concentrated in a much smaller duration t1=1/B. The power of the compressed pulse is therefore increased in the ratio
            T      1              t      1        =            BT      1        .  
Now, in the case of the use of certain coded pulses, for example using Barker codes, the standard detection procedures, applied to a fraction of the pulse received, cause degraded detection and degraded accuracy, since the code is incomplete. This degradation may give rise, for example, to multiple correlation spikes leading to position ambiguities or to a large decline in the power of the pulses and therefore to a loss in effectiveness of the radar.
Systems are also known which use pulses containing a code whose pulse repetition period is reduced by a factor N with respect to the initial signal. The pulse duration is reduced by the same factor N, thereby also dividing by N the length of the zone in which the radar cannot locate any object. However with these modifications alone, the system and the signal become inoperative since it becomes ambiguous distance-wise in a zone in which the initial signal was not ambiguous distance-wise.
To limit this ambiguity, a solution is known which is to emit a succession of N pulses of different frequencies. The frequency spacing of these N pulses must be such that the intercorrelation function for these pulses is low enough that it is possible to say without ambiguity from which pulse an echo originates. In the simplest of the known realizations, the reception is narrowband, that is to say the bandwidth of the receiver is matched to the band of a pulse. To receive the echoes originating from the various frequencies, the central reception frequency is simply shifted. The drawback of this simple procedure is that, at a given instant, only 1/N of the instrumented domain can be received. To receive the whole of the instrumented domain, the receiver has to be successively recentred on the N frequencies. This gives rise:                Either to a lengthening of the time necessary for the surveillance of the entire domain in terms of distance, if a constant observation time is kept for a given frequency;        Or to a reduction in the observation time for a given pseudo-ambiguity if it is desired to preserve a constant observation time for the totality of the domain in terms of distance (this gives rise to a loss of sensitivity).        
In a more advanced realization, the reception is wideband, that is to say the bandwidth of the receiver covers the totality of the spectrum corresponding to the N frequencies. The signals received in the N frequencies are received simultaneously and assigned to the right frequency by virtue of appropriate frequency filtering (analogue or digital).
The latter realization makes it possible to counter the problem of the zone in which the radar cannot locate any object, this zone being significant at short range. On the other hand, it creates a new problem, namely the appearance of N−2 zones in which the radar cannot locate any object, in the central part of the distance domain. To solve this problem, it is necessary to emit, over several observation sequences, a signal whose PRI is made to vary slightly from sequence to sequence so as to displace the blind central zone. However this solution does not allow optimal performance in the central zone.